Cordierite aluminum magnesium titanate  compositions and ceramic articles  comprising same

ABSTRACT

Disclosed are ceramic bodies comprised of composite cordierite aluminum magnesium titanate ceramic compositions and methods for the manufacture of same.

This application claims the benefit of U.S. Provisional Application No.60/817,723, filed Jun. 30, 2006, entitled “Cordierite Aluminum MagnesiumTitanate Compositions and Ceramic Articles Comprising Same.”

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityto U.S. patent application Ser. No. 12/305,767, filed on Dec. 19, 2008,the content of which is relied upon and incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ceramic compositions and, moreparticularly, to composite ceramic compositions comprised of cordieritealuminum magnesium titanate.

2. Technical Background

Refractory materials with low thermal expansion, and consequently highthermal shock resistance, are used in applications such as catalyticconverter substrates and diesel particulate filters where high thermalgradients exist during use. One of the best materials for theseapplications is cordierite due to its low thermal expansion, highmelting point, and low cost. In the diesel particulate filter area, ithas been recognized that higher heat capacity is desirable for improvingdurability of filters during regeneration (Hickman SAE). A material witha high volumetric heat capacity is desirable in order to lower thevolume of material necessary to absorb a given amount of heat. Lessmaterial volume is desirable because this can reduce pressure drop inthe exhaust stream and increase the open volume for ash storage.However, low thermal expansion is still required. Aluminum titanate isone of the few materials that can be made with low thermal expansion andalso has higher volumetric heat capacity than cordierite.

Aluminum titanate (AT) and composites containing large fractions ofaluminum titanate have several disadvantages. First, pure aluminumtitanate is metastable below about 1200° C. Second, the thermalexpansion of AT is only low when the grain size is large and microcracksform during cooling in the kiln. These large grains and microcracks tendto make the material mechanically weak. Third, as a consequence of themicrocracks, the thermal expansion curve can have very large hysteresis,leading to very high values of instantaneous thermal expansion,especially on cooling. Fourth, the firing temperature of AT-basedcomposites is typically high, usually above 1400° C. Finally, AT hasbeen shown to exhibit very high thermal cycling growth which can beexaggerated by the presence of alkali elements.

To slow down the decomposition rate, additives such as mullite, MgTi₂O₅,and Fe₂TiO₅ can be added to the aluminum titanate. MgTi₂O₅ tends to slowthe decomposition rate in reducing conditions and only slows the rate inoxidizing conditions at high levels (>10%). Fe₂TiO₅ tends to slow thedecomposition rate in oxidizing conditions and increase thedecomposition rate in reducing conditions (U.S. Pat. No. 5,153,153,1992).

Second phases such as mullite have been added to AT to increase thestrength of the composite body, because microcracking generally does notoccur between mullite crystals. Mullite also is a good choice because italso has a fairly high volumetric heat capacity. Other second phaseshave also been used in AT composites, including alkali and alkalineearth feldspars. However, mullite and alkali feldspars have a higherthan optimum thermal expansion.

In an effort to provide a composite AT ceramic body having improvedstrength while maintaining a low CTE, cordierite would be a betterchoice than mullite as a second phase because cordierite has a lowercoefficient of thermal expansion than does mullite. However, cordieriteand pure aluminum titanate are not in thermodynamic equilibrium at anytemperature. The provision of a cordierite and AT based compositeceramic having low CTE, high strength, and good thermal stability wouldrepresent an advancement in the state of the art. The present inventionprovides such a body.

SUMMARY OF THE INVENTION

The present invention relates to composite ceramic compositionscomprising cordierite aluminum magnesium titanate. In one aspect, itprovides a ceramic article comprising a first crystalline phasecomprised predominantly of a solid solution of aluminum titanate andmagnesium dititanate and a second crystalline phase comprisingcordierite. In one embodiment, the solid solution phase of aluminumtitanate and magnesium dititanate exhibits a pseudobrookite crystallinestructure. In another embodiment, the ceramic article comprises a totalporosity % P greater than 40% by volume.

In another aspect the invention includes a diesel particulate filtercomprised of the inventive ceramic compositions summarized above. In oneembodiment, the diesel particulate filter comprises a honeycombstructure having a plurality of axially extending end-plugged inlet andoutlet cells.

In yet another aspect the invention provides a method for manufacturingthe inventive composite cordierite aluminum magnesium titanate ceramicarticles of the present invention. The method generally comprises firstproviding an inorganic batch composition comprising a magnesia source, asilica source, an alumina source, and a titania source. The inorganicbatch composition is then mixed together with one or more processingaid(s) selected from the group consisting of a plasticizer, lubricant,binder, pore former, and solvent, to form a plasticized ceramicprecursor batch composition. The plasticized ceramic precursor batchcomposition can be shaped or otherwise formed into a green body,optionally dried, and subsequently fired under conditions effective toconvert the green body into a ceramic article.

BRIEF DESCRIPTION OF THE FIGURES

The invention is further described below with reference to the appendeddrawings, wherein:

FIG. 1 depicts the stable combination of phases as a function oftemperature and composition along the pseudo-binary join betweenaluminum titanate (Al₂TiO₅) and cordierite (Mg₂Al₄Si₅O₁₈).

FIG. 2 depicts the phase relations at 1300° C. in the ternary sectionwith endpoints of magnesium dititanate, aluminum titanate, andcordierite within the quaternary MgO—Al₂O₃—TiO₂—SiO₂ system.

FIG. 3 illustrates the change in length as a function of time at 1100°C. for a control aluminum titanate ceramic composition and for acomposition in the cordierite/mullite/pseudobrookite region of the phasediagram.

FIG. 4 demonstrates the change in the 25-1000° C. coefficient of thermalexpansion for a control aluminum titanate ceramic composition and thecordierite/mullite/pseudobrookite composition of Table 1 after 100 hoursat temperatures of from 950 to 1250° C.

FIG. 5 shows representative data for pressure drop as a function of sootloading for a cordierite/mullite/pseudobrookite ceramic wall flow filtermade in accordance with the invention.

FIG. 6 depicts the microstructure of an inventive body withapproximately 55 grams/liter of alumina washcoat.

DETAILED DESCRIPTION

As briefly summarized above, in one embodiment the present inventionprovides a composite ceramic body comprising a first crystalline phasecomprised predominantly of a solid solution of aluminum titanate andmagnesium dititanate (MgTi₂O₅—Al₂TiO₅) and a second crystalline phasecomprising cordierite. The compositions of the ceramic bodies can becharacterized as comprising, when expressed on weight percent oxidebasis: from 4 to 10% MgO; from 40 to 55% Al₂O₃; from 25 to 35% TiO₂;from 5 to 25% SiO₂, from 0 to 5% Y₂O₃, from 0 to 5% La₂O₃, and from 0 to3% Fe₂O₃. In these or still other embodiments, the compositions of theceramic bodies of the invention are expressed in terms of weightfractions of oxides and oxide combinations to comprise, on an oxidebasis, a(Al₂TiO₅)+b(MgTi₂O₅)+c(2MgO.2Al₂O₃.5SiO₂)+d(3Al₂O₃2SiO₂)+e(MgO.Al₂O₃)+f(2MgO.TiO₂)+g(Y₂O₃)+h(La2O₃)+i(Fe₂O₃.TiO₂)+j(TiO₂),wherein a, b, c, d, e, f, g, h, i, and j are weight fractions of eachcomponent such that (a+b+c+d+e+f+g+h+i+j)=1.00. To that end, the weightfraction of each component can be in the respective ranges as follows:0.3≦a≦0.75, 0.075≦b≦0.3, 0.02≦c≦0.5, 0.0≦d≦0.4, 0.0≦e≦0.25, 0.0≦f≦0.1,0.0≦g≦0.05, 0.0≦h≦0.05, 0.0≦i≦0.05, and 0.0≦j≦0.20. It will berecognized that the oxides and oxide combinations used to define theoxide compositions of these ceramics will not necessarily be present inthe ceramic bodies as the corresponding free oxides or crystal phases,other than as those crystal phases are specifically identified herein ascharacteristic of these ceramics.

The solid solution aluminum titanate and magnesium dititanate phasepreferably exhibits a pseudobrookite crystal structure. To that end, thecomposition of the pseudobrookite phase can depend upon the processingtemperature as well as the overall bulk composition of the ceramic and,as such, can be determined by an equilibrium condition. However, in oneembodiment, the composition of the pseudobrookite phase comprises fromapproximately 20% to 35% MgTi₂O₅ by weight. Still further, while thetotal volume of the pseudobrookite phase can also vary, in anotherembodiment, the total volume is preferably in the range of from 50 to 95volume % of the overall ceramic composition.

Optionally, the composite ceramic body can further comprise one or morephases selected from the group consisting of mullite, sapphirine, atitania polymorph such as rutile or anatase, and a spinel solid solution(MgAl₂O₄—Mg₂TiO₄). When present, the composition of the spinel phasewill also depend on processing temperatures and overall bulkcomposition. However, in one embodiment, the spinel phase can compriseat least about 95% MgAl₂O₄.

Still further, the ceramic composition can also optionally comprise oneor more metal oxide sintering aid(s) or additives provided to lower thefiring temperature and broaden the firing window required to form theceramic composition. A sintering aid can, for example, be present in anamount of from 0 to 5 weight percent of the total composition and caninclude, for example, one or more metal oxides such as Fe₂TiO₅, Y₂O₃,and La₂O₃. In one embodiment, yttrium oxide (Y₂O₃) and/or lanthanumoxide (La₂O₃) has been found to be a particularly good sinteringadditive when added in an amount of between 0.5 and 4.0 wt. %, and morepreferably between 1.0 and 2.0 wt. %. To that end, the yttrium oxide orlanthanide oxide may be present as the oxide phase, or may form a newphase with one or more of the other metal oxide constituents of theceramic body. Similarly, iron oxide from a suitable iron source, presentas ferrous or ferric oxide or in combination with other oxides, e.g., asFe₂TiO₅, can be present in some embodiments in an amount, calculated asFe₂TiO₅, of from 0 to 3 weight % Fe₂TiO₅. The presence of Fe₂TiO₅ can beuseful for slowing decomposition in oxidizing atmospheres. When bothFe₂TiO₅ and a spinel phase are present in the ceramic body, the spinelsolid solution can also additionally contain ferrous and/or ferric ironin the solid solution.

According to a particular embodiment of the present invention, theceramic body comprises approximately 10 to 25 wt. % cordierite,approximately 10 to 30 wt. % mullite, approximately 50 to 70 wt. % of apseudobrookite phase consisting predominantly of an Al₂TiO₅—MgTi₂O₅solid solution, and approximately 0.5 to 3.0 wt. % Y₂O₃ addition.

The ceramic bodies of the present invention can in some instancescomprise a relatively high level of total porosity. For example, bodiescomprising a total porosity, % P, of at least 40%, at least 45%, or evenat least 50%, as determined by mercury porosimetry, can be provided.

In addition to the relatively high total porosities, ceramic bodies ofthe present invention can also comprise a relatively narrow pore sizedistribution evidenced by a minimized percentage of relatively fineand/or relatively large pore sizes. To this end, relative pore sizedistributions can be expressed by a pore fraction which, as used herein,is the percent by volume of porosity, as measured by mercuryporosimetry, divided by 100. For example, the quantity d₅₀ representsthe median pore size based upon pore volume, and is measured inmicrometers; thus, d₅₀ is the pore diameter at which 50% of the openporosity of the ceramic sample has been intruded by mercury. Thequantity d₉₀ is the pore diameter at which 90% of the pore volume iscomprised of pores whose diameters are smaller than the value of d₉₀;thus, d₉₀ is also equal to the pore diameter at which 10% by volume ofthe open porosity of the ceramic has been intruded by mercury. Stillfurther, the quantity d₁₀ is the pore diameter at which 10% of the porevolume is comprised of pores whose diameters are smaller than the valueof d₁₀; thus, d₁₀ is equal to the pore diameter at which 90% by volumeof the open porosity of the ceramic has been intruded by mercury. Thevalues of d₁₀ and d₉₀ are also expressed in units of micrometers.

The median pore diameter, d₅₀, of the pores present in the instantceramic articles can, in one embodiment, be at least 10 μm, morepreferably at least 14 μm, or still more preferably at least 16 μm. Inanother embodiment, the median pore diameter, d₅₀, of the pores presentin the instant ceramic articles do not exceed 30 μm, and more preferablydo not exceed 25 μm, and still more preferably do not exceed 20 μm. Instill another embodiment, the median pore diameter, d₅₀, of the porespresent in the instant ceramic articles can be in the range of from 10μm to 30 μm, more preferably from 18 μm to 25 μm, even more preferablyfrom 14 μm to 25 μm, and still more preferably from 16 μm to 20 μm. Tothis end, a combination of the aforementioned porosity values and medianpore diameter values can provide low clean and soot-loaded pressure dropwhile maintaining useful filtration efficiency when the ceramic bodiesof the present invention are used in diesel exhaust filtrationapplications.

The relatively narrow pore size distribution of the inventive ceramicarticles can, in one embodiment, be evidenced by the width of thedistribution of pore sizes finer than the median pore size, d₅₀, furtherquantified as pore fraction. As used herein, the width of thedistribution of pore sizes finer than the median pore size, d₅₀, arerepresented by a “d_(factor)” or “d_(f)” value which expresses thequantity (d₅₀−d₁₀)/d₅₀. To this end, the ceramic bodies of the presentinvention can comprises a d_(factor) value that does not exceed 0.50,0.40, 0.35, or even that does not exceed 0.30. In some preferredembodiments, the d_(factor) value of the inventive ceramic body does notexceed 0.25 or even 0.20. To this end, a relatively low d_(f) valueindicates a low fraction of fine pores, and low values of d_(f) can bebeneficial for ensuring low soot-loaded pressure drop when the inventiveceramic bodies are utilized in diesel filtration applications.

The relatively narrow pore size distribution of the inventive ceramicarticles can in another embodiment also be evidenced by the width of thedistribution of pore sizes that are finer or coarser than the medianpore size, d₅₀, further quantified as a pore fraction. As used herein,the width of the distribution of pore sizes that are finer or coarserthan the median pore size, d₅₀, are represented by a “d_(breadth)” or“d_(B)” value which expresses the quantity (d₉₀−d₁₀)/d₅₀. To this end,the ceramic structure of the present invention in one embodimentcomprises a d_(b) value that is less than 1.50, less than 1.25, lessthan 1.10, or even less than 1.00. In some especially preferredembodiments, the value of d_(b) is not more than 0.8, more preferablynot greater than 0.7, and even more preferably not greater than 0.6. Arelatively low value of d_(b) can provide a relatively higher filtrationefficiency and higher strength for diesel filtration applications.

The ceramic bodies of the present invention can, in another embodiment,exhibit a low coefficient of thermal expansion resulting in excellentthermal shock resistance (TSR). As will be appreciated by one ofordinary skill in the art, TSR is inversely proportional to thecoefficient of thermal expansion (CTE). That is, a ceramic body with lowthermal expansion will typically have higher thermal shock resistanceand can survive the wide temperature fluctuations that are encounteredin, for example, diesel exhaust filtration applications. Accordingly, inone embodiment, the ceramic articles of the present invention arecharacterized by having a relatively low coefficient of thermalexpansion (CTE) in at least one direction and as measured bydilatometry, that is less than or equal to about 25.0×10⁻⁷/° C., lessthan or equal to 20.0×10⁻⁷/° C.; less than or equal to 15.0×10⁻⁷/° C.,less than or equal to 10.0×10⁻⁷/° C., or even less than or equal to8.0×10⁻⁷/° C., across the temperature range of from 25° C. to 1000° C.

Still further, it should be understood that embodiments of the presentinvention can exhibit any desired combination of the aforementionedproperties. For example, in one embodiment, it is preferred that the CTE(25-1000° C.) does not exceed 12×10⁻⁷/° C. (and preferably not more than10×10⁻⁷/° C.), the porosity % P is at least 45%, the median porediameter is at least 14 μm (and preferably at least 18 μm), and thevalue of d_(f) is not more than 0.35 (and preferably not more than0.30). It is further preferred that such exemplary ceramic bodiesexhibit a value of d_(b) that does not exceed 1.0, and more preferablythat does not exceed 0.85, and still more preferably that does notexceed 0.75.

The ceramic bodies of the present invention can have any shape orgeometry suitable for a particular application. In high temperaturefiltration applications, such as diesel particulate filtration, forwhich the inventive bodies are especially suited, it is preferred thebodies to have a multicellular structure, such as that of a honeycombmonolith. For example, in an exemplary embodiment, the ceramic body cancomprise a honeycomb structure having an inlet and outlet end or face,and a multiplicity of cells extending from the inlet end to the outletend, the cells having porous walls. The honeycomb structure can furtherhave cellular densities from 70 cells/in² (10.9 cells/cm²) to 400cells/in² (62 cells/cm²). A portion of the cells at the inlet end orface end can, in one embodiment, be plugged with a paste having same orsimilar composition to that of the honeycomb structure, as described inU.S. Pat. No. 4,329,162 which is herein incorporated by reference. Theplugging is only at the ends of the cells which is typically to a depthof about 5 to 20 mm, although this can vary. A portion of the cells onthe outlet end but not corresponding to those on the inlet end areplugged. Therefore, each cell is plugged only at one end. A preferredarrangement is to have every other cell on a given face plugged as in acheckered pattern.

This plugging configuration allows for more intimate contact between theexhaust stream and the porous wall of the substrate. The exhaust streamflows into the substrate through the open cells at the inlet end, thenthrough the porous cell walls, and out of the structure through the opencells at the outlet end. Filters of the type herein described are knownas “wall flow” filters since the flow paths resulting from alternatechannel plugging require the exhaust being treated to flow through theporous ceramic cell walls prior to exiting the filter.

The present invention also provides a method of manufacturing theinventive composite cordierite aluminum magnesium titanate ceramicarticles from a ceramic forming precursor batch composition comprised ofcertain inorganic powdered raw materials. Generally, the method firstcomprises providing an inorganic batch composition comprising a magnesiasource, a silica source, an alumina source, and a titania source. Theinorganic batch composition is then mixed together with one or moreprocessing aid(s) selected from the group consisting of a plasticizer,lubricant, binder, pore former, and solvent, to form a plasticizedceramic precursor batch composition. The plasticized ceramic precursorbatch composition can be shaped or otherwise formed into a green body,optionally dried, and subsequently fired under conditions effective toconvert the green body into a ceramic article.

The magnesia source can, for example and without limitation, be selectedfrom one or more of MgO, Mg(OH)₂, MgCO₃, MgAl₂O₄, Mg₂SiO₄, MgSiO₃,MgTiO₃, Mg₂TiO₄, MgTi₂O₅, talc, and calcined talc. Alternatively, themagnesia source can be selected from one or more of forsterite, olivine,chlorite, or serpentine. Preferably, the magnesia source has a medianparticle diameter that does not exceed 35 μm, and preferably that doesnot exceed 30 μm. To this end, as referred to herein, all particlediameters are measured by a laser diffraction technique such as by aMicrotrac particle size analyzer.

The alumina source can, for example and without limitation, be selectedfrom an alumina-forming source such as corundum, Al(OH)₃, boehmite,diaspore, a transition alumina such as gamma-alumina or rho-alumina.Alternatively, the alumina source can be a compound of aluminum withanother metal oxide such as MgAl₂O₄, Al₂TiO₅, mullite, kaolin, calcinedkaolin, phyrophyllite, kyanite, etc. In one embodiment, the weightedaverage median particle size of the alumina sources is preferably in therange of from 10 μm to 60 μm, and more preferably in the range of from20 μm to 45 μm. In still another embodiment, the alumina source can be acombination of one or more alumina forming sources and one or morecompounds of aluminum with another metal oxide.

The titania source can, in addition to the compounds with magnesium oralumina described above, be provided as TiO₂ powder.

The silica source can be provided as a SiO₂ powder such as quartz,cryptocrystalline quartz, fused silica, diatomaceous silica, low-alkalizeolite, or colloidal silica. Additionally, the silica source can alsobe provided as a compound with magnesium and/or aluminum, including forexample, cordierite, chlorite, and the like. In still anotherembodiment, the median particle diameter of the silica source ispreferably at least 5 μm, more preferably at least 10 μm, and still morepreferably at least 20 μm.

As described above, one or more metal oxide sintering aid(s) oradditives can optionally be added to the precursor batch composition tolower the firing temperature and broaden the firing window required toform the ceramic composition. The sintering aid can, for example, bepresent in an amount of from 0 to 5 weight percent of the totalcomposition and can include, for example, one or more a metal oxidessuch as Fe₂TiO₅, Y₂O₃, and La₂O₃. In one embodiment, yttrium oxide(Y₂O₃) and/or lanthanide oxide (La₂O₃) has been found to be aparticularly good sintering additive when added in an amount of between0.5 and 4.0 wt. %, and more preferably between 1.0 and 2.0 wt. %.Similarly, an addition of Fe₂TiO₅ can be useful for slowingdecomposition in oxidizing atmospheres when added in an amount of from 0to 3 weight %.

Still further, the precursor composition can, if desired, contain apore-forming agent to tailor the porosity and pore size distribution inthe fired body for a particular application. A pore former is a fugitivematerial which evaporates or undergoes vaporization by combustion duringdrying or heating of the green body to obtain a desired, usually higherporosity and/or coarser median pore diameter. A suitable pore former caninclude, without limitation, carbon; graphite; starch; wood, shell, ornut flour; polymers such as polyethylene beads; waxes; and the like.When used, a particulate pore former can have a median particle diameterin the range of from 10 μm to 70 μm, and more preferably from 20 μm to50 μm.

The inorganic ceramic forming batch components, along with any optionalsintering aid and/or pore former, can be intimately blended with aliquid vehicle and forming aids which impart plastic formability andgreen strength to the raw materials when they are shaped into a body.When forming is done by extrusion, most typically a cellulose etherbinder such as methylcellulose, hydroxypropyl methylcellulose,methylcellulose derivatives, and/or any combinations thereof, serve as atemporary organic binder, and sodium stearate can serve as a lubricant.The relative amounts of forming aids can vary depending on factors suchas the nature and amounts of raw materials used, etc. For example, thetypical amounts of forming aids are about 2% to about 10% by weight ofmethyl cellulose, and preferably about 3% to about 6% by weight, andabout 0.5% to about 1% by weight sodium stearate or stearic acid, andpreferably about 0.6% by weight. The raw materials and the forming aidsare typically mixed together in dry form and then mixed with water asthe vehicle. The amount of water can vary from one batch of materials toanother and therefore is determined by pre-testing the particular batchfor extrudability.

The liquid vehicle component can vary depending on the type of materialused in order to impart optimum handling properties and compatibilitywith the other components in the ceramic batch mixture. Typically, theliquid vehicle content is usually in the range of from 20% to 50% byweight of the plasticized composition. In one embodiment, the liquidvehicle component can comprise water. In another embodiment, dependingon the component parts of the ceramic batch composition, it should beunderstood that organic solvents such as, for example, methanol,ethanol, or a mixture thereof can be used as the liquid vehicle.

Forming or shaping of the green body from the plasticized precursorcomposition may be done by, for example, typical ceramic fabricationtechniques, such as uniaxial or isostatic pressing, extrusion, slipcasting, and injection molding. Extrusion is preferred when the ceramicarticle is of a honeycomb geometry, such as for a catalytic converterflow-through substrate or a diesel particulate wall-flow filter. Theresulting green bodies can be optionally dried, and then fired in a gasor electric kiln or by microwave heating, under conditions effective toconvert the green body into a ceramic article. For example, the firingconditions effective to convert the green body into a ceramic articlecan comprise heating the green body at a maximum soak temperature in therange of from 1250° C. to 1450° C., more preferably in the range of from1300° C. to 1350° C., and maintaining the maximum soak temperature for ahold time sufficient to convert the green body into a ceramic article,followed by cooling at a rate sufficient not to thermally shock thesintered article.

Still further, the effective firing conditions can comprise heating thegreen body at a first soak temperature in the range of from 1240 to1350° C. (preferably 1270 to 1330° C.), holding the first soaktemperature for a period of from 2 to 10 hours (preferably 4 to 8hours), then heating the body at a second soak temperature in the rangeof from 1270 to 1450° C. (preferably 1300-1350° C.), and holding thesecond soak temperature for a period of from 2 to 10 hours (preferably 4to 8 hours), again followed by cooling at a rate sufficient not tothermally shock the sintered article.

To obtain a wall-flow filter, a portion of the cells of the honeycombstructure at the inlet end or face are plugged, as known in the art. Theplugging is only at the ends of the cells which is typically to a depthof about 1 to 20 mm, although this can vary. A portion of the cells onthe outlet end but not corresponding to those on the inlet end areplugged. Therefore, each cell is plugged only at one end. The preferredarrangement is to have every other cell on a given face plugged in acheckered pattern.

A greater understanding of the findings underlying the present inventioncan be obtained with reference to phase equilibrium diagrams for theMgO—Al₂O₃—TiO₂—SiO₂ system, prepared by the present inventors. It willof course be recognized that many of the boundaries between phase fieldsincluded in such diagrams represent the results of equilibriumcalculations and extrapolation rather than actual phase analyses. Whilethe phase fields themselves have been confirmed with experiments, theprecise temperatures and compositions representing boundaries betweenphase fields are approximate. In any case, the phase diagram of FIG. 1depicts the stable combination of phases as a function of temperatureand composition along the pseudo-binary join between aluminum titanate(Al₂TiO₅) and cordierite (Mg₂Al₄Si₅O₁₈). Essentially, this diagramindicates that mixtures of cordierite and AT at high temperature willtend to form other phases, including mullite, titania, liquid, and asolid-solution phase with the pseudobrookite crystal structure.

Two significant features can be derived from a study of this diagram.First, in order for the pseudobrookite phase to be in equilibrium withcordierite there is a general restriction on the composition of thesolid-solution, in particular, pure AT will tend to not exist inequilibrium with cordierite. FIG. 2 depicts the phase relations at 1325°C. in the ternary section with endpoints of magnesium dititanate,aluminum titanate, and cordierite within the quaternaryMgO—Al₂O₃—TiO₂—SiO₂ system, showing that the pseudobrookite phase inequilibrium with cordierite contains at least about 25 wt % magnesiumdititanate at this temperature. Second, FIG. 1 shows that a liquidappears in the diagram at fairly low temperatures (˜1390 C, although thelowest eutectic liquid in this system exists well below this).

EXAMPLES

The invention is further described below with respect to certainexemplary and specific embodiments thereof, which are illustrative onlyand not intended to be limiting. In accordance with some of theexamples, a series of inventive ceramic articles is prepared having thegeneral inorganic batch composition as provided in Table 1, in terms ofthe weight percentages of the end-member phases, and as provided inTable 2, expressed in terms of the weight percentages of the singlecomponent oxides, excluding any sintering additive.

TABLE 1 Formula Name Weight % Al₂TiO₅ AT 40 MgTi₂O₅ MT2 20 Al₆Si₂O₁₃Mullite 25 Mg₂Al₄Si₅O₁₈ Cordierite 15

TABLE 2 Metal Oxide Weight % MgO 6.10 Al₂O₃ 45.61 TiO₂ 33.54 SiO₂ 14.76

Tables 3 to 9 provide data for the inventive examples fabricatedaccording to the general composition of Tables 1 and 2. Listed are theraw materials, pore formers, and sintering aid (median particlediameters in parentheses) used to make the samples. The examplesprovided have been made by mulling component powders with water and anorganic binder, followed by extrusion, drying, and firing. All extrudedsamples were wrapped in foil and hot-air dried. Samples weresubsequently fired in an electric kiln by heating at 60° C./hr to afirst soak temperature and holding for 6 hours, then heated at 60° C./hrto a second soak temperature and held for another 6 hours. Soaktemperatures are also provided in Tables 3 to 9. These examples will bediscussed further below. All measurements, except where noted, were oncellular pieces with 200 cells per square inch and 16 mil wallthicknesses. All samples were fired in air in electric furnaces unlessotherwise noted. CTE was measured parallel to the honeycomb channels bydilatometry. Porosity and pore size distribution were derived frommercury porosimetry measurements.

Also provided in Tables 3 to 9 is the “maximum ΔL at 1000° C,” definedas the value of ΔL/L at 1000° C. due to thermal expansion upon heating athermal expansion specimen to 1000° C. from room temperature, minus theminimum value of ΔL/L that occurs during cooling of a thermal expansionspecimen from 1000° C. to that lower temperature at which the minimumvalue of ΔL/L exists. The values of maximum ΔL at 1000° C. are reportedin Tables 3 to 9 as a percentage value; thus, for example, a maximum ΔLat 1000° C. of 0.15% is equal to a ΔL value of 0.15×10⁻², which is alsoequivalent to 1500 ppm, or 1500×10⁻⁶ inches/inch. The value of maximumΔL at 1000° C. is a measure of the degree of hysteresis between thethermal expansion curves (ΔL/L vs. temperature) during heating andcooling.

In addition to measurement of the property data in Tables 3 to 9,several special measurements were made to characterize the thermalstability of the inventive materials, and to determine their pressuredrop behavior when used as a diesel particulate filter.

The thermal stability (decomposition rate) was assessed by two methods.In the first method, specimens of the inventive body and of a controlaluminum titanate composition were held at 1100° C. and their lengthsmonitored over time for up to 100 hours. Decomposition of thepseudobrookite phase is accompanied by a decrease in volume (shrinkage,or negative length change). The results, shown in FIG. 3, demonstratethe superior stability of the inventive body, for which thedecomposition rate of the pseudobrookite phase is at least 10 timesslower than for the control aluminum titanate composition. In a secondmethod to assess decomposition rate, the CTE of the inventivecomposition and control aluminum titanate composition was measuredbefore and after isothermally holding the sample for 100 hours attemperatures of from 950 to 1250° C. Because the decomposition of thepseudobrookite phase reduces the amount of microcracking, raising theCTE, the increase in CTE after heat treatment is an indication of thedegree of decomposition. The results are shown in FIG. 4, anddemonstrate the improved thermal stability of the inventive bodies.

The pressure drops of clean and soot-loaded filters formed of arepresentative cordierite aluminum magnesium titanate ceramic accordingto the invention and an aluminum titanate control ceramic were measuredon the bare and catalyzed filters. The filter of the invention was of300/12 cell geometry. Washcoating was done after a conventionalpreliminary polymer solution passivation, using AL-20 colloidal aluminafor the washcoat. Representative results of such pressure drop testingare set forth in FIG. 5, wherein the % increase in pressure drop afterwashcoating is found to be lower for the filter of the invention thanfor the control aluminum titanate filter. The microstructure of thewashcoated filter thus tested is shown in FIG. 6.

The data in Tables 3 to 9 further illustrate some of the exemplaryranges in properties that can be achieved with the inventive ceramicbodies of the current invention. Examples 1 to 7 in Table 3 representthe baseline quaternary three-phase composition (Tables 1 and 2) with nosintering additive. These examples show that low thermal expansion (6 to20×10⁻⁷/° C.) can be achieved with porosities (44-52%) and median porediameters (15-27 μm) appropriate for application as a diesel particulatefilter. The d_(f) values range from 0.24 to 0.45. The optimum top firingtemperature for these compositions is approximately 1355 to 1360° C. Thecoarser alumina used in Examples 4-7 results in higher pore size andlower firing shrinkage.

Examples 8 to 15 in Table 4 illustrate that the addition of about 2 wt.% Y₂O₃ to the base composition of Examples 1-3 allows a lower firingtemperature, between 1290-1320° C., and a broader range of firingtemperatures with high porosity (41-50%) and low thermal expansion (10to 14×10⁻⁷/° C.). Median pore diameters are 16 to 22 μm, and d_(f)values are reduced to 0.17 to 0.31. The change in shrinkage with firingtemperature is also lower. This allows a wider process window to achievethe desired properties. The optimum firing temperature is approximately1310° C.

Examples 16 to 22 in Table 5 demonstrate that the addition of only about1% Y₂O₃ super-addition to the base composition of Examples 1-3 reducesthe firing temperature to 1310-1350° C., with the optimum beingapproximately 1320° C. The lower level of additive results in a firingtemperature and firing process window that is intermediate between thebasic quaternary composition and that for 2 wt. % additive. Physicalproperties are still excellent for a diesel particulate filterapplication.

Example 23 in Table 6 shows that the use of a finer, 10 μm, aluminapowder results in a smaller pore size, slightly higher shrinkage, andslightly higher thermal expansion compared with Examples 8-15.

Examples 24 to 30 in Table 6 illustrate that the use of an aluminapowder with coarser particle size results in larger pores, lower thermalexpansion, and lower shrinkage compared to Examples 8-15. Thiscomposition has a very stable firing process window because of thecoarse alumina and 2 wt. % yttria. This was a 2-inch diameter extrusiondried in a dielectric oven.

Examples 31 to 37 In Table 7 demonstrate compositions in which all ofthe magnesium was supplied by talc, and in which the alumina is of afiner particle size (˜18 micron MPS). All have 1.9 wt. % yttriaaddition. Example 31 uses 15% potato starch. Example 32 uses 15% cornstarch, which gives smaller pores but a very narrow pore sizedistribution. Example 33 contains 30% graphite and still provides auseful median pore diameter (12 μm) and narrow pore size distribution(d_(f)=0.29). Example 34 utilizes a mixture of corn starch and graphiteto achieve good properties. Example 35 shows that coarser alumina andtalc result in lower firing shrinkage on the same firing schedule andraise the pore size relative to Example 32. Example 36 made with greenbean starch yields 15 micron pores and a very narrow pore-sizedistribution. Example 37 using potato starch shows that coarser aluminaand talc raise the pore size relative to Example 31.

TABLE 3 Example Number 1 2 3 4 5 6 7 Alumina A (24) 44.76 44.76 44.76 —— — — Alumina B (42) — — — 44.76 44.76 44.76 44.76 Alumina C (10) — — —— — — — Alumina D (18) — — — — — — — Silica A (25) — — — — — — — SilicaB (23) 8.65 8.65 8.65 8.65 8.65 8.65 8.65 Titania A (0.5) 33.85 33.8533.85 33.85 33.85 33.85 33.85 Magnesia A (1.2) 3.01 3.01 3.01 3.01 3.013.01 3.01 Talc A (5.0) 9.73 9.73 9.73 9.73 9.73 9.73 9.73 Talc B (14.4)— — — — — — — Talc C (23) — — — — — — — Y₂O₃ — — — — — — — Graphite A(35) 25.00 25.00 25.00 25.00 25.00 25.00 25.00 Corn Starch (17) — — — —— — — Potato Starch (49) — — — — — — — First Soak Temperature (° C.)1320 1330 1335 1325 1330 1335 1340 First Soak Time (hours) 6 6 6 6 6 6 6Second Soak Temperature (° C.) 1347 1357 1362 1352 1357 1362 1367 SecondSoak Time (hours) 6 6 6 6 6 6 6 Length Change after Firing (%) 1.7 −1.1−2.1 1.3 0.9 0.2 −0.5 CTE, 25-800° C. (10⁻⁷/° C.) 16.2 9.8 6.3 8.7 4.93.0 6.6 CTE, 25-1000° C. (10⁻⁷/° C.) 19.5 12.6 9.1 12.1 8.4 6.2 10.0Maximum ΔL at 1000° C. (%) 0.22 0.19 0.17 0.17 0.15 0.15 0.15 % Porosity52.1 50.6 44.0 52.1 51.5 51.5 44.6 d₅₀ (microns) 14.5 15.1 16.1 23.223.5 22.5 27.3 (d₅₀ − d₁₀)/d₅₀ 0.45 0.44 0.27 0.42 0.38 0.38 0.24 (d₉₀ −d₁₀)/d₅₀ 1.16 1.08 1.01 1.26 1.09 1.45 1.19

TABLE 4 Example Number 8 9 10 11 12 13 14 15 Alumina A (24) 43.90 43.9043.90 43.90 43.90 43.90 43.90 43.90 Alumina B (42) — — — — — — — —Alumina C (10) — — — — — — — — Alumina D (18) — — — — — — — — Silica A(25) — — — — — — — — Silica B (23) 8.48 8.48 8.48 8.48 8.48 8.48 8.488.48 Titania A (0.5) 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19Magnesia A (1.2) 2.96 2.96 2.96 2.96 2.96 2.96 2.96 2.96 Talc A (5.0)9.54 9.54 9.54 9.54 9.54 9.54 9.54 9.54 Talc B (14.4) — — — — — — — —Talc C (23) — — — — — — — — Y₂O₃ 1.94 1.94 1.94 1.94 1.94 1.94 1.94 1.94Graphite A (35) 30.00 30.00 30.00 30.00 30.00 30.00 30.00 30.00 CornStarch (17) — — — — — — — — Potato Starch (49) — — — — — — — — FirstSoak Temperature (° C.) 1275 1285 1290 1295 1305 1315 1320 1330 FirstSoak Time (hours) 6 6 6 6 6 6 6 6 Second Soak Temperature (° C.) 13021312 1317 1322 1332 1342 1347 1357 Second Soak Time (hours) 6 6 6 6 6 66 6 Length Change after Firing (%) −1.9 −2.8 −2.6 −3.5 −4.3 −4.6 −4.9−6.5 CTE, 25-800° C. (10⁻⁷/° C.) 6.8 7.4 6.3 7.4 7.5 11.2 9.6 8.3 CTE,25-1000° C. (10⁻⁷/° C.) 10.2 10.8 10.0 10.8 11.3 13.9 13.5 11.7 MaximumΔL at 1000° C. (%) 0.17 0.17 0.18 0.16 0.17 0.18 0.18 0.17 % Porosity50.4 48.3 49.3 47.2 45.7 43.9 41.5 41.1 d₅₀ (microns) 16.0 17.0 16.618.0 20.1 22.0 20.2 21.6 (d₅₀ − d₁₀)/d₅₀ 0.31 0.27 0.30 0.23 0.21 0.17 —0.17 (d₉₀ − d₁₀)/d₅₀ — — 0.75 0.60 0.71 0.79 — 0.87

TABLE 5 Example Number 16 17 18 19 20 21 22 Alumina A (24) 44.33 44.3344.33 44.33 44.33 44.33 44.33 Alumina B (42) — — — — — — — Alumina C(10) — — — — — — — Alumina D (18) — — — — — — — Silica A (25) — — — — —— — Silica B (23) 8.56 8.56 8.56 8.56 8.56 8.56 8.56 Titania A (0.5)33.52 33.52 33.52 33.52 33.52 33.52 33.52 Magnesia A (1.2) 2.99 2.992.99 2.99 2.99 2.99 2.99 Talc A (5.0) 9.63 9.63 9.63 9.63 9.63 9.63 9.63Talc B (14.4) — — — — — — — Talc C (23) — — — — — — — Y₂O₃ 0.98 0.980.98 0.98 0.98 0.98 0.98 Graphite A (35) 30.00 30.00 30.00 30.00 30.0030.00 30.00 Corn Starch (17) — — — — — — — Potato Starch (49) — — — — —— — First Soak Temperature (° C.) 1285 1290 1295 1305 1315 1320 1330First Soak Time (hours) 6 6 6 6 6 6 6 Second Soak Temperature (° C.)1312 1317 1322 1332 1342 1347 1357 Second Soak Time (hours) 6 6 6 6 6 66 Length Change after Firing (%) −0.9 −0.3 −1.1 −2.6 −3.7 −3.9 −5.1 CTE,25-800° C. (10⁻⁷/° C.) 11.3 11.6 8.4 8.4 7.2 6.3 10.8 CTE, 25-1000° C.(10⁻⁷/° C.) 14.6 15.3 11.8 11.7 10.9 9.7 14.3 Maximum ΔL at 1000° C. (%)0.19 0.20 0.17 0.18 0.17 0.18 0.18 % Porosity 51.3 51.9 50.5 51.1 43.943.9 42.5 d₅₀ (microns) 14.5 13.9 15.3 16.0 18.1 18.5 20.1 (d₅₀ −d₁₀)/d₅₀ 0.39 0.45 0.35 0.33 0.23 0.22 0.17 (d₉₀ − d₁₀)/d₅₀ 1.17 0.800.84 0.75 0.66 0.67 0.93

TABLE 6 Example Number 23 24 25 26 27 28 29 30 Alumina A (24) — — — — —— — — Alumina B (42) — 43.90 43.90 43.90 43.90 43.90 43.90 43.90 AluminaC (10) 43.71 — — — — — — — Alumina D (18) — — — — — — — — Silica A (25)— 8.48 8.48 8.48 8.48 8.48 8.48 8.48 Silica B (23) 8.49 — — — — — — —Titania A (0.5) 33.36 33.19 33.19 33.19 33.19 33.19 33.19 33.19 MagnesiaA (1.2) 2.96 2.96 2.96 2.96 2.96 2.96 2.96 2.96 Talc A (5.0) 9.54 9.549.54 9.54 9.54 9.54 9.54 9.54 Talc B (14.4) — — — — — — — — Talc C (23)— — — — — — — — Y₂O₃ 1.94 1.94 1.94 1.94 1.94 1.94 1.94 1.94 Graphite A(35) 25.00 30.00 30.00 30.00 30.00 30.00 30.00 30.00 Corn Starch (17) —— — — — — — — Potato Starch (49) — — — — — — — — First Soak Temperature(° C.) 1290 1250 1260 1270 1280 1290 1300 1290 First Soak Time (hours) 66 6 6 6 6 6 6 Second Soak Temperature (° C.) 1317 1277 1287 1297 13071317 1327 1317 Second Soak Time (hours) 6 6 6 6 6 6 6 6 Length Changeafter Firing (%) −3.1 −1.9 −2.0 −1.6 −2.1 −1.8 −1.8 — CTE, 25-800° C.(10⁻⁷/° C.) 7.2 9.0 6.8 6.3 4.1 3.3 3.0 5.1 CTE, 25-1000° C. (10⁻⁷/° C.)11.0 12.0 10.4 9.9 7.7 6.8 6.4 7.9 Maximum ΔL at 1000° C. (%) 0.18 0.170.16 0.16 0.15 0.13 0.14 0.13 % Porosity 47.0 46.6 48.3 45.6 47.6 46.044.0 46.4 d₅₀ (microns) 10.7 19.8 21.1 20.2 22.8 23.9 23.9 21.9 (d₅₀ −d₁₀)/d₅₀ 0.27 0.29 0.30 0.29 0.26 0.28 0.22 0.55 (d₉₀ − d₁₀)/d₅₀ 0.830.78 1.12 0.72 0.93 0.89 0.91 1.73

TABLE 7 Example Number 31 32 33 34 35 36 37 Alumina A (24) — — — — 43.5143.51 43.51 Alumina B (42) — — — — — — — Alumina C (10) — — — — — — —Alumina D (18) 43.57 43.57 43.57 43.57 — — — Silica A (25) 2.68 2.682.68 2.68 2.65 2.65 2.65 Silica B (23) — — — — — — — Titania A (0.5)33.01 33.01 33.01 33.01 32.94 32.94 32.94 Magnesia A (1.2) — — — — — — —Talc A (5.0) — — — — — — — Talc B (14.4) 18.81 18.81 18.81 18.81 — — —Talc C (23) — — — — 18.97 18.97 18.97 Y₂O₃ 1.93 1.93 1.93 1.93 1.93 1.931.93 Graphite A (35) — — 30.00 10.00 — — — Green Bean Starch — — — — —15.00 — Corn Starch (17) — 15.00 — 10.00 15.00 — — Potato Starch (49)15.00 — — — — — 15.00 First Soak Temperature (° C.) 1285 1285 1285 12851285 1285 1285 First Soak Time (hours) 6 6 6 6 6 6 6 Second SoakTemperature (° C.) 1312 1312 1312 1312 1312 1312 1312 Second Soak Time(hours) 6 6 6 6 6 6 6 Length Change after Firing (%) −1.1 −1.6 −2.5 −1.6−0.3 0.1 0.3 CTE, 25-800° C. (10⁻⁷/° C.) 6.0 7.1 6.1 6.7 8.9 7.4 7.2CTE, 25-1000° C. (10⁻⁷/° C.) 9.2 10.4 9.1 9.8 12.1 10.9 10.5 Maximum ΔLat 1000° C. (%) 0.15 0.15 0.16 0.15 0.16 0.15 0.15 % Porosity 48.4 44.446.7 44.7 46.5 47.1 49.1 d₅₀ (microns) 15.7 10.3 12.2 10.9 12.5 14.519.1 (d₅₀ − d₁₀)/d₅₀ 0.39 0.20 0.29 0.25 0.23 0.22 0.32 (d₉₀ − d₁₀)/d₅₀0.87 0.52 0.74 0.70 1.12 0.72 0.94

TABLE 8 Example Number 38 39 40 41 42 43 44 Alumina A (24) 42.17 42.1745.61 45.61 45.61 45.61 42.17 Silica B (23) 11.93 11.93 18.26 18.2611.48 11.48 15.13 Titania B (8-16) 39.13 39.13 29.62 29.62 37.24 37.2435.55 Magnesia A (1.2) 6.77 6.77 6.51 6.51 5.67 5.67 7.15 First SoakTemperature (° C.) 1400 1375 1400 1375 1400 1375 1400 First Soak Time(hours) 8.00 8.00 8.00 8.00 8.00 8.00 8.00 Length Change after Firing(%) −9 0 −7 1 2 1 −3 CTE, 25-800° C. (10⁻⁷/° C.) 1 7 — — — — — CTE,25-1000° C. (10⁻⁷/° C.) 5 10 5 13 14 21 8 Maximum ΔL at 1000° C. (%)0.19 0.19 0.17 0.16 0.16 0.23 0.12 % Porosity — 40.6 19.38 40.28 32.3342.33 26.88 d₅₀ (microns) — 7.1 12.69 14.99 23.01 7.59 15.24

TABLE 9 Example Number 45 46 47 48 49 50 Alumina A (24) 42.17 42.1742.17 44.00 48.00 48.00 Silica B (23) 15.13 8.49 8.49 6.84 9.42 12.83Titania B (8-16) 35.55 41.42 41.42 43.52 37.84 33.99 Magnesia A (1.2)7.15 7.92 7.92 5.64 4.74 5.18 First Soak Temperature (° C.) 1375 14001375 1400 1400 1400 First Soak Time (hours) 8.00 8.00 8.00 8.00 8.008.00 Length Change after Firing (%) 2 −11 −1 −7 3 −2 CTE, 25-800° C.(10⁻⁷/° C.) — — — — — — CTE, 25-1000° C. (10⁻⁷/° C.) 20 5 15 4 13 11Maximum ΔL at 1000° C. (%) 0.23 0.13 0.22 0.16 0.18 0.17 % Porosity37.30 14.63 36.63 24.65 34.22 35.26 d₅₀ (microns) 10.26 0.47 7.14 6.839.46 14.42

1-20. (canceled)
 21. A ceramic article comprising a first crystallinephase comprised predominantly of a solid solution of aluminum titanateand magnesium dititanate and a second crystalline phase comprisingcordierite, the article having a composition, as expressed in weightpercent on an oxide basis of from 4 to 10% MgO; from 40 to 55% Al₂O₃;from 25 to 35% TiO₂; from 5 to 25% SiO₂, and a metal oxide sinteringaid, wherein the ceramic article comprises a total porosity % P greaterthan 40% by volume and a coefficient of thermal expansion, as measuredbetween 25-1000° C., of less than or equal to 15×10⁻⁷/° C.
 22. Theceramic article of claim 21, wherein the metal oxide sintering aidcomprises at least one of a yttrium oxide and a lanthanide oxide. 23.The ceramic article of claim 21 having a composition expressed on anoxide basis of: a(Al₂TiO₅)+b(MgTi₂O₅)+c(2MgO.2Al₂O₃.5SiO₂)+d(3Al₂O₃.2SiO₂)+e(MgO.Al₂O₃)+f(2MgO.TiO₂)+g(X)+i(Fe₂O₃.TiO₂)+j(TiO₂), wherein a,b, c, d, e, f, g, i, and j are weight fractions of each component suchthat (a+b+c+d+e+f+g+i+j)=1.00, wherein X is a metal oxide sintering aid,and wherein 0.3≦a≦0.75, 0.075≦b≦0.3, 0.02≦c≦0.5, 0.0≦d≦0.4, 0.0≦e≦0.25,0.0≦f≦0.1, 0.001≦g≦0.05, 0.0≦i≦0.05, and 0.0≦j≦0.2.
 24. The ceramicarticle of claim 23, wherein the metal oxide sintering aid comprises atleast one of a yttrium oxide and a lanthanide oxide.
 25. The ceramicarticle of claim 21 having a composition, as expressed in weight percenton an oxide basis: of from 5 to 10% MgO; from 40 to 50% Al₂O₃; from 30to 35% TiO₂; and from 10 to 20% SiO₂.
 26. The ceramic article of claim21, wherein the metal oxide is present, on a weight percent oxide basis,in an amount in the range of from greater than 0.1 to 5 weight %relative to the total weight of the inorganic batch composition.
 27. Aceramic article comprising a first crystalline phase comprisedpredominantly of a solid solution of aluminum titanate and magnesiumdititanate and a second crystalline phase comprising cordierite, thearticle having a composition, as expressed in weight percent on an oxidebasis of from 4 to 10% MgO; from 40 to 55% Al₂O₃; from 25 to 35% TiO₂;from 5 to 25% SiO₂, and at least one metal oxide sintering aid, whereinthe ceramic article comprises a total porosity % P greater than 40% byvolume.
 28. The ceramic article of claim 27, wherein the ceramic articlecomprises a coefficient of thermal expansion, as measured between25-1000° C., of less than or equal to 15×10⁻⁷/° C.
 29. The ceramicarticle of claim 27, wherein the ceramic article comprises a totalporosity % P at least 50% by volume.
 30. The ceramic article of claim27, wherein the metal oxide sintering aid comprises at least one of ayttrium oxide and a lanthanide oxide.